This application is a Continuation-in-Part of U.S. Patent Application Ser. No. 08/068,560, filed May 28, 1993, now U.S. Pat. No. 5,504,785 for "DIGITAL RECEIVER FOR VARIABLE DATA RATE COMMUNICATIONS", assigned to the assignee of this application.
This invention relates to the reception and detection of communications signals, and more particularly, to the reception of such signals transmitted with variable data rates.
In a communications system, data is formatted onto a carrier signal and transmitted by a transmitter. After the signal travels through some intervening medium, it is received and decoded by the receiver. Ideally, the waveform of the data would remain unchanged during the communications process. In practice, however, the waveform is distorted and corrupted by its passage both through the electronic circuitry of the transmitter and the receiver, and through the medium. An important feature of the receiver is the processing of the received signal to determine the actual content of the data even though the transmitting signal has become distorted and corrupted during the transmission and reception process.
For example, in a typical satellite communications system a data signal is created at one location on the earth, encoded onto a radio signal, and transmitted to a satellite in synchronous orbit above the earth. The satellite retransmits the received signal to another location on the earth, where it is received and demodulated. The data-carrying signal passes through several electronic systems, as much as 44,000 miles of free space, and twice through the atmosphere, and in all of these portions of the transmission it is subject to external interference and distortions.
Historically, the signal has been transmitted and processed entirely by analog techniques. More recently, digital signal processing techniques are being adopted because they permit more precise determination of the data content of the signal. In digital signal processing, the receiver has a conventional analog tuner that receives and shifts the signal to a lower frequency. The receiver thereafter samples the received analog signal to form a digital pulse train or signal. The digitized signal is further processed to extract the data content.
This known approach works well for the condition that the transmitted signal has a fixed data rate known to the receiver, which permits the receiver to be configured for the characteristics of the known transmitted signal. In other instances, however, it is desirable to vary the data rate of the transmitter for various reasons. For example, a single satellite channel may be used to carry many different types of data signals, some of which are transmitted at a high data rate and some of which are transmitted at a low data rate. In another example, if the satellite channel carries a compressed video signal, it may be desirable to vary the data rate depending upon the type of programming being carried. A video feed of a conference could be transmitted at a lower data rate than a video feed of a sports event, for example, due to the differences in the speed of the action.
Several problems arise in complex communications systems having multiple channels, where the data rate is variable in each channel. As the data rate of the signal in a channel changes, the sampling rate of the sampler in the receiver must change in order to satisfy the Nyquist sampling criterion. The sampler is normally synchronized to the data rate, and its sampling rate changes to track changes in the data rate. However, for other reasons it is strongly preferred not to change the sampling rate in an arbitrary fashion so as to accommodate changes in the data rate. A second problem is that it is difficult to achieve initial acquisition of the data of the received information, when the channel is first activated.
There is a need for a digital receiver system that is operable at variable rates, particularly in a multichannel communications system. The present invention fulfills this need, and further provides related advantages.